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Full text of "Text Book Of Mechanical Engineering"

42                               Steel Casting.

While speaking of scrap, we cannot do better than endeavour
to understand the advantage or otherwise of remelting. We have
before said that remelting is a disadvantage. It is true that the
iron becomes purer as regards the elimination of graphite, ac-
quiring a 'whiter appearance, with, at the same time, increased
strength and closeness of grain; but, on account of other im-
purities, it is no longer as tough as before, and its ultimate
extension is therefore decreased. Unwin says: ' Remelting im-
proves the strength, but if repeated too long the tensile and
transverse strengths suffer, though the crushing strength and
hardness increase.' (See Appendix //.,/. 779.)

For chilled castings, a strong iron, as Nos. 3 and 4, is needed,
because the chilling weakens the metal.

Malleable castings require a pure mottled iron, or at least one
having very little grey mixed with it; for if the particles of graphite
present in coarse grey iron are taken away in the furnace, honey-
combing will result.

Girders and columns must have a strong and elastic mixture;
cylinders should be treated for hardness as well as strength, and
therefore require as much white iron as convenient; pulleys need
a soft mixture, such as a large proportion of No. i with a little of
No. 3 for strength.

Steel Casting requires no explanation. Its conversion from
iron will be treated in a subsequent chapter. The only difference
in the foundry is, that in order to prevent 'honeycombing,7 which
has been a great trouble ever since steel castings were first used,
great care has to be exercised, and even then many castings are
wasted, while brittleness is only prevented by slow annealing for
over a week or a fortnight of time. (See Appendix /., /. 747.)

Sir Joseph Whitworth introduced a method of compressing the
steel while in the molten ingot by powerful hydraulic pressure,
in order to prevent this troublesome honeycombing, the only
objection to his process being considerably increased cost. (See
also bottom of p. 82, and Appendix ///.,/, 790.)